1. Field of the Invention
The present invention relates to a method for selectively precipitating metal ions from solutions containing these ions.
2. Description of Related Art
Because the solubilities of the sulphides of various metals are often remarkably different, the separation of metals on the basis of their solubility is a widespread method of separation. The commonest sulphide reagent is hydrogen sulphide, H.sub.2 S. Hydrogen sulphide is a good reagent because it is a weak acid and thus the S.sup.-- concentration is easily adjusted by choosing a suitable pH value in the precipitation. The precipitation may take place for example in the following order: Cu, Zn and Co, which are precipitated from their mixed solution by adjusting the pH value to be roughly 1.0 in the Cu precipitation, whereafter the pH value is raised up to about 2.5 and the ZnS is precipitated, and finally the pH value is raised up to 5 and CoS is precipitated. If the initial solution contains for instance Cu, Zn and Co, 10 g/l each, the distribution of copper, zinc and cobalt in the precipitate in percentages is roughly as follows:
______________________________________ Cu Zn Co ______________________________________ CuS 98 2 0 ZnS 2 88 1 CoS 0 10 99 ______________________________________
This degree of separation is sufficient for utilization in industrial processes, but far from complete.
Moreover, heavy metals such as Cu, Zn, Co, Ni, Pb and Cd can easily be separated from solutions containing large amounts of Fe.sup.2+, Mn, Mg, Na and Ca, because the sulphides of the latter metals are much more easily soluble.
Other possible sulphide reagents are Na.sub.2 S, Ca(HS).sub.2, and (NH.sub.4).sub.2 S. These reagents behave in a more or less similar fashion to hydrogen sulphide, except that their selectivity is somewhat poorer due to difficulties in avoiding local fluctuation of the pH values and hence fluctuation in the S.sup.-- concentrations.
There are also known methods for precipitating Ni from solutions containing Co by employing elemental sulphur and iron powder as reagents.
The above mentioned methods have, however, some drawbacks. H.sub.2 S is an expensive reagent to produce, it is corrosive and highly poisonous, and therefore while it is being used, care must be continuously taken that the system is absolutely closed. Iron powder, which is used in the other methods, is also an expensive reagent.
In the prior art of chemistry it is also known that some heavy metal oxides react with elemental sulphur in aqueous solutions at high temperatures, about 250.degree. C., in an autoclave and form metal sulphides. The salts of some metals, such as Ag and Hg, may in aqueous solutions form sulphides while reacting with elemental sulphur. When the sulphidizer is elemental sulphur, the reaction goes in principle as follows: EQU 4 MeO+4 S.degree..fwdarw.3 MeS.sub.(s) +MeSo.sub.4(aq) 1.
i.e. the sulphur is disproportioned.